The present invention relates to a negative pressure booster equipment to be used for brake booster and the like, and more particularly, to a negative pressure booster equipment, in which two or more power pistons are arranged in tandem.
As a brake booster using negative pressure, a tandem type brake booster is conventionally known, in which two power pistons are arranged in tandem, for example, as described in the Japanese Provisional Utility Model Publication No. 63-53860, in order to obtain big braking power by small brake pedal force.
This booster equipment comprises a space formed by a front shell and a rear shell, and the space is divided into two chambers by center plate. Each of these chambers is divided into a constant pressure chamber and a variable pressure chamber by said two power pistons.
In this tandem type brake booster equipment, when the brake pedal is depressed for braking purpose, a control valve is switched over, and the atmospheric air is introduced through a passage into two variable pressure chambers divided by two power pistons. Two power pistons are then operated by the atmospheric pressure of the air thus introduced. By the action of these power pistons, the master cylinder is operated to generate the braking hydraulic pressure and to perform braking operation. In this case, the maximum braking hydraulic pressure thus generated is higher than that of the brake booster equipment operated by a single power piston because the piston of the master cylinder is operated by two power pistons.
In this tandem type brake booster equipment, the power piston is furnished with a diaphragm, and the inner peripheral bead of the diaphragm is fixed on the valve body, which accommodates said control valve. Thus, said passage where the air flows is opened to the variable pressure chamber at a point between the fixed portion of inner peripheral bead of the diaphragm and said center plate.
However, when the passage is opened to the variable pressure chamber between inner peripheral bead and the center plate, the gap between the center plate and the fixed portion of the bead must be maintained at least to some extent. This requires longer overall length of the equipment.